A variety of woody resin molded bodies and molding methods therefor have been developed, and hollow molded bodies or foam molded bodies composed of woody resins have been developed to reduce weight. When nails are driven or screws are tightened in such hollow molded bodies, however, it is necessary to take account of the hollow portion, and market requirements are not necessarily fully satisfied in terms of secondary processability. Furthermore, in a case where hollow molded bodies are used as exterior materials, the applicational operations are troublesome because the end surfaces of the hollow portion have to be closed with a cover to improve appearance.
Meanwhile, in the case of foam molded bodies, when the foaming ratio is increased to reduce the molded body weight, mechanical strength is reduced. Conversely, when the foaming ratio is set low to maintain strength, light weight for molded body cannot be obtained. Because of this dilemma, the foam molded bodies also do not necessarily fulfill market requirements.
As methods for resolving simultaneously the problems associated with the molded body weight and secondary processability, a molding method for initially molding a woody resin molded body having a hollow structure and then injecting a foamed resin into the hollow portion thereof, and a method for co-extrusion molding a non-foamed layer of a woody resin composition on the outside and a high-expanded foam layer of identical or different resin on the inside, have recently been suggested. However, productivity is difficult to improve by the former method because the process involves two stages. The latter method is technologically difficult because it involves co-extrusion molding, and in addition, productivity is difficult to improve compared with the usual extrusion molding.
Patent Documents 1 and 2 disclose molded bodies having a structure in which the density in the cross section of the molding increases from the central portion to the surface, these molded bodies being advantageous in terms of weight, secondary processability such as nail driving and screw tightening, and product strength. However, because these molded bodies are formed using a profile extrusion die having a core body, weld lines caused by a fixing member serving to fix the core body to the die are inevitably formed in the surface layer. As a result, market requirements are not sufficiently satisfied in terms of aesthetic properties and strength. Furthermore, in some cases deterioration of weather resistance or extension-shrinkage movement in linear expansion cause disaggregation of weld lines on the aesthetic surface, thereby decreasing strength and rigidity. In particular, water penetrates into the disaggregated weld lines in exterior applications, dust is retained therein, mold appears, and beauty of appearance is lost. Accordingly, a demand has been created for a method for resolving these problems.    Patent Document 1: Japanese Patent No. 3626112.    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-314372.